this invention relates generally to turbine engine shrouds disposed about rotating articles and to their assemblies about rotating blades. More particularly, it relates to air cooled gas turbine engine shroud segments and to shroud assemblies, for example used in the turbine section of a gas turbine engine, especially segments made of a low ductility material.
Typically in a gas turbine engine, a plurality of stationary shroud segments are assembled circumferentially about an axial flow engine axis and radially outwardly about rotating blading members, for example about turbine blades, to define a part of the radial outer flowpath boundary over the blades. In addition, the assembly of shroud segments is assembled in an engine axially between such axially adjacent engine members as nozzles and/or engine frames. As has been described in various forms in the gas turbine engine art, it is desirable to avoid leakage of shroud segment cooling air radially inwardly and engine flowpath fluid radially outwardly through separations between circumferentially adjacent shroud segments and between axially adjacent engine members. It is well known that such undesirable leakage can reduce turbine engine operating efficiency. Some current seal designs and assemblies include sealing members disposed in slots in shroud segments. Typical forms of current shrouds often have slots along circumferential and/or axial edges to retain thin metal strips sometimes called spline seals. During operation, such spline seals are free to move radially to be pressure loaded at the slot edges and thus to minimize shroud segment to segment leakage. Because of the usual slot configuration, stresses are generated at relatively sharp edges. However as discussed below, current metallic materials from which the shroud segments are made can accommodate such stresses without detriment to the shroud segment. Examples of U.S. Patents relating to turbine engine shrouds and such shroud sealing include U.S. Pat. No. 3,798,899—Hill; U.S. Pat. No. 3,807,891—McDow et al.; U.S. Pat. No. 5,071,313—Nichols; U.S. Pat. No. 5,074,748—Hagle; U.S. Pat. No. 5,127,793—Walker et al.; and U.S. Pat. No. 5,562,408—Proctor et al.
Metallic type materials currently and typically used to make shrouds and shroud segments have mechanical properties including strength and ductility sufficiently high to enable the shrouds to receive and retain currently used inter-segment leaf or spline seals in slots in the shroud segments without resulting in damage to the shroud segment during engine operation. Generally such slots conveniently are manufactured to include relatively sharp corners or relatively deep recesses that can result in locations of stress concentrations, sometimes referred to as stress risers. That kind of assembly can result in the application of a substantial compressive force to the shroud segments during engine operation. If such segments are made of typical high temperature alloys currently used in gas turbine engines, the alloy structure can easily withstand and accommodate such compressive forces without damage to the segment. However, if the shroud segment is made of a low ductility, relatively brittle material, such compressive loading can result in fracture or other detrimental damage to the segment during engine operation.
Current gas turbine engine development has suggested, for use in higher temperature applications such as shroud segments and other components, certain materials having a higher temperature capability than the metallic type materials currently in use. However such materials, forms of which are referred to commercially as a ceramic matrix composite (CMC), have mechanical properties that must be considered during design and application of an article such as a shroud segment. For example, CMC type materials have relatively low tensile ductility or low strain to failure when compared with metallic materials. Therefore, if a CMC type of shroud segment is manufactured with features such as relatively sharp corners or deep recesses to receive and hold a fluid seal, such features can act as detrimental stress risers. Compressive forces developed at such stress risers in a CMC type segment can be sufficient to cause failure of the segment.
Generally, commercially available CMC materials include a ceramic type fiber for example SiC, forms of which are coated with a compliant material such as BN. The fibers are carried in a ceramic type matrix, one form of which is SiC. Typically, CMC type materials have a room temperature tensile ductility of no greater than about 1%, herein used to define and mean a low ductility material. Generally CMC type materials have a room temperature tensile ductility in the range of about 0.4-0.7%. This is compared with metallic materials currently used as shrouds, and supporting structure or hanger materials, that have a room temperature tensile ductility of at least about 5%, for example in the range of about 5-15%. Shroud segments made from CMC type materials, although having certain higher temperature capabilities than those of a metallic type material, cannot tolerate the above described and currently used type of compressive forces generated in slots or recesses for fluid seals. Therefore, a shroud segment and assembly of shroud segments configured to receive and hold an inter-segment fluid seal without generating detrimental stress can enable advantageous use of low ductility shroud segments with fluid seals retained therebetween without operating damage to the brittle segments.